Printed circuit board design for high speed application

ABSTRACT

A printed circuit board (PCB) is disclosed. The PCB includes a substrate have a top surface and a bottom surface. A first conductive layer is disposed on the top surface of the substrate. The first conductive layer comprises a first signal net and a second signal net. An outermost insulating layer is disposed on the top surface of the substrate to cover the substrate and the first conductive layer. The outmost insulating layer comprises an opening to expose a portion of the second signal net. A second conductive layer is disposed on the outermost insulating layer and substantially covering at least a portion of the first signal net. The second conductive layer is filled into the opening to electrically connect to the second signal net which is able to provide one of a ground potential and a power potential.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. application Ser. No. 13/408,062 filed Feb. 29, 2012, now U.S. Pat. No. 9,949,360 B2, which itself claims the benefit of U.S. provisional application No. 61/451,283, filed Mar. 10, 2011. The above-mentioned applications are included in their entirety herein by reference.

BACKGROUND 1. Field of the Invention

The invention relates to a printed circuit board and more particularly to a printed circuit board design with good signal quality.

2. Description of the Related Art

Typically, a printed circuit board (PCB) is a layered insulating structure with internal and external traces/planes that allows electronic devices/components on the printed circuit board to be electrically connected to each other and to the outside environment. Printed circuit boards are the most commonly used packaging medium for electronic circuits. Due to the demand for high performance systems, packaging density and propagation speeds have increased, to force the technological development of the printed circuit boards to evolve from being single-sided to double-sided or multilayered.

Notwithstanding, along with rapid advances in printed circuit board technology, electromagnetic interference (EMI) problems have increased in severity. When electronic devices/components operate in high speed and are allocated in the PCB with a high device density, noise may be generated and affect the operation of other electronic devices. In a good printed circuit board design, signal delay, distortion and crosstalk noise are minimized. Crosstalk is a kind of noise induced primarily by the electromagnetic coupling between signal lines and degrades signal quality. In printed circuit boards, crosstalk occurs by the electrical coupling between nearby signal traces. As more and more functions are integrated on a chip, more circuit traces of the printed circuit board are demanded, and thus the coupling between nearby signal traces have become greater, introducing noise and false signals into systems.

Additionally, referring to FIGS. 8A and 8B, FIG. 8A shows a plan view of a conventional two-layered printed circuit board 300 which shows routing of power and signal planes. FIG. 8B shows a cross section along line 8B-8B′ of FIG. 8A. The conventional printed circuit board 300 has a top layer on a top surface 102 of a substrate 100, covered by a solder mask layer 126. The top layer comprises power traces 108 a and 108 b and a signal trace 112. The conventional printed circuit board 300 also has a bottom layer covered by a solder mask layer 130 comprising a ground plane 140 on a bottom surface 103 of the substrate 100. For descriptive convenience, the solder mask layer 126 is not shown in FIG. 8A. The power traces 108 a and 108 b are used for providing power potential, and the signal trace 112 is used for transmitting signal or data. As shown in FIG. 8A, the signal trace 112 is substantially placed along a second direction 304 which may form a power transmission barrier between the adjacent power traces 108 a and 108 b which is substantially placed along a first direction 302, which is not parallel to the second direction 304. As shown in FIG. 8B, in order to transmit power potential between the adjacent power traces 108 a and 108 b, a conductive trace 108 c is formed on a bottom surface 103 of the substrate 100. The conductive trace 108 c is respectively electrically connected to the power traces 108 a and 108 b by via plugs 134 through the substrate 100, and isolated from a split ground plane 140 by a gap 150. As shown in FIG. 8A, the signal trace 112 is across over the gap 150 surrounding the conductive trace 108 c. When signals, especially high-speed signals, are transmitted along the signal trace 112, however, the current return path of the high-speed signals not only remains under the signal trace 112, but also transmit along the gap 150 directly under the signal trace 112. The long current return path may generate an undesired magnetic field vertical to the directions 302 and 304, and thus increases the coupling coefficient between adjacent signal traces and exacerbates electromagnetic interference (EMI) problems. A multi-layered PCB, which separates power, signal and ground planes in various layers, may be used to mitigate the aforementioned problems, but increasing layers of the PCB will also raise the manufacturing cost of the printed circuited board.

Accordingly, there is a need to develop printed circuit board designs which are capable of mitigating the aforementioned problems.

SUMMARY

Printed circuit boards are provided. An exemplary embodiment of a printed circuit board comprises a substrate having a top surface and a bottom surface. A first conductive layer is disposed on the top surface of the substrate. The first conductive layer comprises a first signal net and a second signal net. An outermost insulating layer is disposed on the top surface of the substrate to cover the substrate and the first conductive layer. The outmost insulating layer comprises an opening to expose a portion of the second signal net. And, a second conductive layer is disposed on the outermost insulating layer and substantially covering at least a portion of the first signal net. The second conductive layer is filled into the opening to electrically connect to the second signal net which is able to provide one of a ground potential and a power potential.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 illustrates a cross section of an exemplary embodiment of a one-layered printed circuit board according to the invention;

FIG. 2A illustrates a cross section of an exemplary embodiment of a two-layered printed circuit board according to the invention;

FIG. 2B illustrates a cross section of another exemplary embodiment of a two-layered printed circuit board according to the invention;

FIG. 3 illustrates a cross section of an exemplary embodiment of a four-layered printed circuit board according to the invention;

FIG. 4 illustrates a cross section of an exemplary embodiment of a six-layered printed circuit board according to the invention;

FIG. 5A illustrates a simulation diagram showing a relationship between near-end crosstalk and frequency for signal traces of two-layered printed circuit boards as the embodiment shown in FIG. 2;

FIG. 5B illustrates a simulation diagram showing a relationship between return loss and frequency for signal traces of two-layered printed circuit boards as the embodiment shown in FIG. 2;

FIGS. 6A to 6C are an exemplary simulation diagram in time domain showing an eye-open of one byte of writing data signals transmitted through a DDR3 DRAM interface;

FIGS. 7A to 7C are an exemplary simulation diagram in time domain showing an eye-open of one byte of writing data signals transmitted through a DDR3 DRAM interface;

FIG. 8A shows a plan view of a conventional two-layered printed circuit board;

FIG. 8B shows a cross section along line 8B-8B′ of FIG. 8A; and

FIG. 9 is a schematic layout diagram showing a germane portion of the printed circuit board including an exemplary memory chip, an exemplary controller chip, and high-speed bus therebetween according to another embodiment of the invention.

DETAILED DESCRIPTION

The following description encompasses the fabrication and the purpose of the invention. It can be understood that this description is provided for the purpose of illustrating the fabrication and the use of the invention and should not be taken in a limited sense. In the drawings or disclosure, the same or similar elements are represented or labeled by the same or similar symbols. Moreover, the shapes or thicknesses of the elements shown in the drawings may be magnified for simplicity and convenience. Additionally, the elements not shown or described in the drawings or disclosure are common elements which are well known in the art.

In the following various exemplary embodiments, the printed circuit board may comprise net, traces, strip lines, or micro-strip which are made of conductive material to electrically connect various electronic components for transmitting data and/or signals in a high speed manner. Such net, traces, strip lines, or micro-strip may be function as a high-speed signal bus or interface, such as memory interfaces, high-definition multimedia interface (HDMI), serial advanced technology attachment (SATA) interface, universal serial bus (USB), Ethernet interface or low voltage differential signaling (LVDS) interface. Referring to FIG. 1, a cross section of an exemplary embodiment of a one-layered printed circuit board according to one embodiment of the invention is illustrated. In the embodiment, the printed circuit board 200 comprises a substrate 202 made of FR-1, FR-2, FR-4, CEM-1 or CEM-3 material. The substrate 202 has a top surface 202 a and a bottom surface 202 b. A first conductive layer 203 comprising nets, micro-strips, traces, or strip lines for electrically transmitting signals is formed on the top surface 202 a. The first conductive layer 203 is made of conductive material comprising copper, aluminum, copper alloy or aluminum alloy. An outermost insulating layer 204 is formed on the top surface 202 a of the substrate 202 to substantially cover the top surface 202 a and the first conductive layer 203. In the embodiment, the outermost insulating layer 204 may comprise a solder mask or other suitable dielectric materials, e.g. isolation film, isolation tape or pre-preg.

In one embodiment, the first conductive layer 203 comprises a first signal net 203 a and a second signal net 203 b and 203 c. In one embodiment, the first signal net 203 a are electrically connected between electronic components for transmitting signal and/or data. The second signal net 203 b and 203 c are electrically connected to a ground potential to serve as a ground net. Alternatively, the second signal net 203 b and 203 c are electrically connected to a power source having a voltage potential to serve as a power net. The first signal net 203 a and second signal net 203 b and 203 c may be patterned to comprise the following structures, either alone or in any combination, including trace, plane, pad, finger, or via hole. The first signal net 203 a and second signal net 203 b and 203 c are substantially covered by the outermost insulating layer 204. In the embodiment of this application, the outermost insulating layer 204 has openings 209 to expose at least a portion of the second signal net 203 b. The exposed portion may comprise a pad, a via hole, a portion of a trace or a portion of a plane of the second signal net 203 b.

In particular, a second conductive layer 207 is disposed on the outermost insulating layer 204 to substantially cover at least a portion of the first conductive layer 203. In the embodiment, the second conductive layer 207 may be formed of solid metal sheet (such as metal foil/tape which comprises Al, Cu, Ag, or Au). In another embodiment, the second conductive layer 207 can be formed from a conductive epoxy or conductive epoxy adhesive (such as conductive silver, copper, or aluminum epoxy or the like). In another embodiment, the second conductive layer 207 can be manufactured from a polymer conductive composite, conductive polymer, conductive composite polymer, or carbon printing.

The second conductive layer 207 is electrically connected to the second signal net 203 b through openings 209 formed in the outermost insulating layer 204 so as to connect to a ground net or a power net. In one embodiment, the openings 209 may be filled with conductive epoxy material or conductive adhesive material so as to form a unity structure with the second conductive layer 207. In another embodiment, a conductive device (not shown), e.g. screw, pin, jumper etc., is placed through the opening 209 to electrically connect the second conductive layer 207 and second signal net 203 b. In addition, in yet another embodiment of the application, the second conductive layer 207 electrically connect to second signal net 203 b through not only the conductive device but also the conductive epoxy material filled into the openings 209. As a result, the second conductive layer 207 can serve as a reference plane to suppress crosstalk noise between first signal nets nearby.

Crosstalk noise which is raised by adjacent first signal net 203 a can be virtually suppressed by the second conductive layer 207 (i.e., the reference plane) in the micro-strip geometry. Note that in order to effectively suppress or eliminate the crosstalk noise between the adjacent first signal net 203 a, in one embodiment, a spacing H1 between the second conductive layer 207 and the first signal net 203 a on the substrate 202 is less than 1 mm, preferable in a range of 50 μm to 350 μm, such that the first signal net 203 a are substantially close to the second conductive layer 207 in a vertical direction with respect to the top surface 202 a of the substrate 202. Additionally, an optional first insulating cap layer 208 may be disposed on the outermost insulating layer 204 to cover the second conductive layer 207. The first insulating cap layer 208 may comprise the same or similar material as that of the outermost insulating layer 204 to protect the underlying second conductive layer 207 from mechanical or chemical damage.

Referring to FIGS. 2A and 2B, which illustrate cross sections of various exemplary embodiments of a two-layered printed circuit board. Elements in FIGS. 2A and 2B that are similar to those in FIG. 1 are labeled with the same reference numbers as in FIG. 1 and are not described again for brevity. In FIG. 2A, the two-layered printed circuit board 400 a comprises an outermost insulating layer 210 disposed on the bottom surface 202 b of the substrate 202. The outermost insulating layer 210 may comprise the same or similar material as that of the outermost insulating layer 204. Moreover, a third conductive layer 309 is disposed on the bottom surface 202 b of the substrate 202 and is embedded in the outermost insulating layer 210. Typically, the third conductive layer 309 may comprise same or similar material as that of the first conductive layer 203 and serves as a ground or power net.

In one embodiment, the second conductive layer 207 may be electrically connected to the third conductive layer 309 through openings 209, second signal net 203 b, and via holes 201. The openings 209 are filled with conductive epoxy material or conductive adhesive material so as to form a unity structure with the second conductive layer 207. As a result, since the second conductive layers 207 and the third conductive layer 309 can serve as reference planes, crosstalk noise between the adjacent first signal nets 203 a of the printed circuit board 400 a can be virtually suppressed by the second conductive layer 207 and the third conductive layer 309 in the micro-strip geometry.

In FIG. 2B, the two-layered printed circuit board 400 b comprises outermost insulating layers 204 and 210 disposed on the top and bottom surfaces 202 a and 202 b of the substrate 202, respectively. A first conductive layer 303 comprises a plurality of non-ground net 303 a (such as signal net and/or power net) and at least one ground net 303 b which are embedded in the outermost insulating layer 204. The plurality of non-ground net 303 a are substantially coplanar with the ground net 303 b. Moreover, the plurality of non-ground net 303 a and the ground net 303 b are formed of the same conductive material, such as copper, aluminum, copper alloy or aluminum alloy.

A third conductive layer 309 comprises at least one non-ground net 309 a (such as signal net or power net) and a ground net 309 b. The non-ground net 309 a and the ground net 309 b are disposed on the bottom surface 202 b of the substrate 202, and are embedded in the outermost insulating layer 210, in which the non-ground net 309 a is placed adjacent to the ground net 309 b. Moreover, that the non-ground net 309 a is isolated from the ground net 309 b by gap 311. Additionally, the ground net 309 b is electrically connected to the ground net 303 b through a via hole 201 b formed in the substrate 202. The non-ground net 309 a is electrically connected to one of the plurality of non-ground net 303 a by a via hole 201 a formed in the substrate 202. The ground net 309 b may be patterned to comprise the following structures, either alone or in any combination, including trace, plane, pad, finger, or via hole.

A long current return path due to the split ground net 309 b may generate an undesired magnetic field, and thus increases the coupling coefficient between adjacent signal nets and exacerbates electromagnetic interference (EMI) problems (S. H. Hall, G. W. Hall, and J. A. McCall, High-Speed Digital System Design. New York, N.Y.: John Wiley & Sons, 2000, ch. 6). Accordingly, in the embodiment, a fourth conductive layer 215 serving as a reference plane is disposed on the outermost insulating layer 210 corresponding to a region/area W of the substrate 202, such that the fourth conductive layer 215 substantially covers the plurality of non-ground net 303 a. The fourth conductive layer 215 may be formed of solid metal sheet (such as metal foil/tape which comprises Al, Cu, Ag, or Au). In another embodiment, the fourth conductive layer 215 can be formed from a conductive epoxy or conductive epoxy adhesive (such as conductive silver, copper, or aluminum epoxy or the like). In another embodiment, the fourth conductive layer 215 can be manufactured from a polymer conductive composite, conductive polymer, conductive composite polymer, or carbon printing.

The fourth conductive layer 215 is electrically connected to the ground net 309 b through openings 221 formed in the outermost insulating layer 210 so as to connect to a ground potential. In one embodiment, the openings 221 may be filled with conductive epoxy material or conductive adhesive material so as to form a unity structure with the fourth conductive layer 215. In another embodiment, a conductive device (not shown), e.g. screw, pin, jumper etc., is placed through the opening 211 to electrically connect the fourth conductive layer 215 and ground net 309 b. In addition, in yet another embodiment of the application, the fourth conductive layer 215 may electrically connect to ground net 309 b through not only the conductive device but also the conductive epoxy material filled into the openings 221.

Coupling coefficient between the adjacent non-ground net 303 a (such as signal net) can be reduced and the EMI can be mitigated by the fourth conductive layer 215 (i.e., the reference plane) in the similar micro-strip geometry. Note that in order to effectively suppress the crosstalk noise (i.e., the coupling coefficient) and mitigate the EMI, a spacing H2 between the fourth conductive layer 215 and the third conductive layer 309 is less than 1 mm, preferable ranged between 50 μm to 350 μm, such that the ground net 309 b and the non-ground net 309 a are substantially close to the fourth conductive layer 215 in a vertical direction with respect to the bottom surface 202 b of the substrate 202. Additionally, an optional second insulating cap layer 216 may be disposed on the outermost insulating layer 210 to cover the fourth conductive layer 215. The second insulating cap layer 216 may comprise the same or similar material as that of the outermost insulating layer 210 to protect the fourth conductive layer 215 from mechanical or chemical damage.

Referring to FIG. 3, which illustrates a cross section of an exemplary embodiment of a four-layered printed circuit board according to the invention. Elements in FIG. 3 that are similar to those in FIGS. 1, 2A and 2B are labeled with the same reference numbers as in FIGS. 1, 2A and 2B and are not described again for brevity. In the embodiment, the printed circuit board 500 comprises a substrate 202. Outermost insulating layer 204 and 210 are disposed on the top and bottom surfaces 202 a and 202 b of the substrate 202, respectively, as the embodiment shown in FIG. 2A or 2B.

Unlike the embodiment shown in FIG. 2A or 2B, third signal net 212 a and fourth signal net 212 b and 212 c are embedded in the outermost insulating layer 210. Moreover, the third signal net 212 a and the fourth signal net 212 b and 212 c are formed of the same conductive layer, such as copper, aluminum, copper alloy or aluminum alloy. The fourth signal net 212 b and 212 c may be configured as traces or planes, and partially with pads, fingers, or via holes. In the embodiment of the application, the outermost insulating layer 210 has openings 410 to expose at least one pad, via hole, a portion of a trace or plane of the fourth signal net 212 b.

A first reference plane 403 (such as a ground or power net) is embedded in the substrate 202 and is electrically connected to the second signal net 203 b by via holes 401 formed in the substrate 202. A second reference plane 404 (such as a power or ground net) is embedded in the substrate 202 and is electrically connected to the fourth signal net 212 b by via holes 402 formed in the substrate 202.

In one embodiment, the second signal net 203 b may be electrically connected to ground potential (i.e., the first reference plane 403 serves as a ground net), while the second signal net 203 c are electrically connected to a power source having a voltage potential. The fourth signal net 212 b may be electrically connected to a power source (i.e., the second reference plane 404 serves as a power net), while the fourth signal net 212 c are electrically connected to the ground potential. Alternatively, the second signal net 203 b may be electrically connected to a power source (i.e., the first reference plane 403 serves as a power net), while the second signal net 203 c are electrically connected to the ground potential. The fourth signal net 212 b may be electrically connected to the ground potential (i.e., the second reference plane 404 serves as a ground net), while the fourth signal net 212 c are electrically connected to a power source.

In particular, the second conductive layers 207 and the fourth conductive layer 215 are disposed on the outermost insulating layers 204 and 210 corresponding to the region/area W, such that the second conductive layer 207 substantially covers the first signal net 203 a, and second signal net 203 b and 203 c, while the fourth conductive layer 215 substantially underlies the third signal net 212 a, and the fourth signal net 212 b and 212 c. In another embodiment, the second signal net 203 c and the fourth signal net 212 c may be outside the region/area W. The fourth conductive layer 215 is electrically connected to the fourth signal net 212 b through openings 410 formed in the outermost insulating layer 210. In one embodiment, the fourth conductive layer 215 is electrically connected to the ground potential when the fourth signal net 212 b and the second reference plane 404 serve as a ground net, while the second conductive layer 207 is electrically connected to a power source when the second signal net 203 b and the first reference plane 403 serve as a power net. In this case, the fourth conductive layer 215 may be electrically connected to ground through the openings 410 by a screw, pin, jumper, conductive adhesive or conductive epoxy. In another embodiment, the fourth conductive layer 215 is electrically connected to the power source when the fourth signal net 212 b and the second reference plane 404 serve as a power net, while the second conductive layer 207 is electrically connected to the ground potential when the second signal net 203 b and the first reference plane 403 serve as a ground net. As a result, the first signal net 203 a and the third signal net 212 a have a strip-line configuration.

Crosstalk noise between the first signal net 203 a and that of between the third signal net 212 a can be virtually suppressed by the respective micro-strip geometry. Also, the spacing H1 and H2 are less than 1 mm, preferable in a range of 50 μm to 350 μm, such that the first signal net 203 a and the second signal net 203 b and 203 c are substantially close to the second conductive layers 207 in a vertical direction with respect to the top surface 202 a of the substrate 202, while the third signal net 212 a and the fourth signal net 212 b and 212 c are substantially close to the fourth conductive layers 215 in a vertical direction with respect to the bottom surface 202 b of the substrate 202. Additionally, an optional second insulating cap layer 216 may be disposed on the outermost insulating layer 210 and covers the fourth conductive layer 215 to protect the fourth conductive layer 215 from mechanical or chemical damage.

Referring to FIG. 4, which illustrates a cross section of an exemplary embodiment of a six-layered printed circuit board according to the invention. Elements in FIG. 4 that are similar to those in FIG. 3 are labeled with the same reference numbers as in FIG. 3 and are not described again for brevity. Comparing to the embodiment of FIG. 3, the six-layered printed circuit board 600 further comprise a plurality of fifth signal net 501 and a plurality of sixth signal net 503 embedded in the substrate 202, in which the plurality of the fifth signal net 501 are disposed between the first reference plane 403 and the outermost insulating layer 204, and the plurality of the sixth signal net 503 are disposed between the second reference plane 404 and the outermost insulating layer 210.

Referring to FIGS. 5A and 5B, FIG. 5A is a simulation diagram showing a relationship between near-end crosstalk (dB) and frequency (GHz) for signal traces of two-layered printed circuit boards as the embodiment shown in FIG. 2, and FIG. 5B is a simulation diagram showing a relationship between return loss and frequency for signal net of two-layered printed circuit board for signal traces of two-layered printed circuit boards as the embodiment shown in FIG. 2. In FIGS. 5A and 5B, the curve A represents signal net of a conventional two-layered printed circuited board without any additional conductive layer disposed on the outermost insulating layer thereon. The curves B and C represent first signal net 203 a of a two-layered printed circuited board 400 a or 400 b with a second conductive layer 207 having a thickness of 100 μm and a spacing H1 of 80 and 110 μm, respectively (as shown in FIG. 2A), and with a fourth conductive layer 215 on an outermost insulating layer 210 having a thickness of 100 μm and a spacing H2 of 80 and 110 μm, respectively (as shown in FIG. 2B). As shown in FIGS. 5A and 5B, the near-end crosstalk and return loss of the signal net during high frequency operations can be reduced by the additional conductive layers 207 and 215.

According to the aforementioned embodiments, since there is an additional conductive layer, which is connected to a ground potential or a power source, disposed on the outermost insulating layer of the printed circuit board and close to the high-speed signal net embedded therein, the crosstalk noise and EMI can be effectively suppressed. Moreover, the formation of such an additional conductive layer (serving as a reference plane) is easy and has a lower fabrication cost when compared with using multi-layered PCBs (such as a 4-layer PCB) with separated power, signal and ground net in various layers. Accordingly, an economic benefit can be obtained by using of a less-layer PCB with the mentioned additional conductive layer for high-speed applications.

Please refer to FIGS. 6A to 6C and Table 1. FIGS. 6A to 6C are an exemplary simulation diagram in time domain showing an eye-open of one byte of writing data signals transmitted through a DDR3 DRAM interface at 1350 Mb/s with an on-die termination of 120 ohms at the DRAM side. Table 1 shows the measurement result of the eye-open shown in FIGS. 6A to 6C. As shown in FIGS. 6A to 6C and Table 1, the larger eye open makes the better voltage and timing margins.

TABLE 1 Type Spacing Conventional Spacing (H1, H2) = Item design (H1, H2) = 80 μm 110 μm V_(IH(dc)) margin (mV) 160 250 270 V_(IL(dc)) margin (mV) 220 250 286 Overshoot (V) 1.61 1.56 1.55 Undershoot (V) −0.11 −0.13 −0.12 Skew (ps, V_(ref) to V_(ref)) 324 244 221 H. Eye-open (ps) 284 394 406

Please refer to FIGS. 7A to 7C and Table 2. FIGS. 7A to 7C are an exemplary simulation diagram in time domain showing an eye-open of one byte of writing data signals transmitted through a DDR3 DRAM interface at 1350 Mb/s without on-die termination at the DRAM side. Table 2 shows the measurement result of the eye-open shown in FIGS. 7A to 7C. As shown in FIGS. 7A to 7C and Table 2, the larger eye open makes the better voltage and timing margins.

TABLE 2 Type Spacing Conventional Spacing (H1, H2) = Item design (H1, H2) = 80 μm 110 μm VIH(dc) margin (mV) 107 280 320 VIL(dc) margin (mV) 157 279 315 Overshoot (V) 2.15 1.92 1.94 Undershoot (V) −0.63 −0.50 −0.52 Skew (ps, Vref to Vref) 529 330 322 H. Eye-open (ps) 153 332 351

Please refer to FIG. 9, and briefly back to FIG. 2A. FIG. 9 is a schematic layout diagram showing a germane portion of the printed circuit board including a first chip 10, a second chip 20, and high-speed interface 30 therebetween within the region W according to another embodiment of the invention, wherein like numeral numbers designate like layers, regions, or elements. It should be noted that all the figures are diagrammatic. Relative dimensions and proportions of parts of the drawings have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings.

In one embodiment, FIG. 2A may be deemed as the cross-sectional view taken along line I-I′ in FIG. 9. However, it is to be understood that the exemplary layout depicted in FIG. 9 may be applicable to other embodiments in this application.

As shown in FIG. 9 and FIG. 2A, the high-speed interface 30 is disposed within the region W and connected between the first chip 10 and the second chip 20 on the printed circuit board 400 a for transmitting high-frequency or high-speed signals. For example, the first chip 10 may be a controller chip (or chipset) such as a memory controller and the second chip 20 may be a DDR3 or DDR4 DRAM chip. According to the embodiment, the high-speed interface 30 is a memory bus. However, it is understood that the first chip 10 and the second chip 20 may comprise other chips and the high-speed interface 30 may comprise other interfaces such as HDMI interface, SATA interface, USB, Ethernet interface or LVDS interface, but is not limited thereto.

As shown in FIG. 9, the exemplary high-speed interface 30 comprises first signal nets 203 a interposed between two adjacent second signal nets 203 b. According to the embodiment, the second signal nets 203 b act as ground guard and are electrically coupled to ground. According to the embodiment, each of the second signal nets 203 b has a line width that is greater than that of each of the first signal nets 203 a. For example, the line width of each of the second signal nets 203 b may be greater than 0.2 mm and the line width of each of the first signal nets 203 a may be smaller than 0.2 mm, but is not limited thereto. The larger width of ground guard 203 b gets the smaller ground impedance. The smaller width of high-speed signal 203 a achieves the higher routing density in the PCB.

According to the embodiment, the second signal nets 203 b are electrically connected to an underlying ground plane through the via holes 201. This ground plane is not explicitly shown in FIG. 9, but can be seen in FIG. 2A as denoted by label 309, in FIG. 3 as denoted by label 403, or in FIG. 4 as denoted by label 403 by way of an example. These via holes 201 are arranged along the lengthwise direction of each of the second signal nets 203 b with a pitch P₁. According to the embodiment, the pitch P₁ may be equal to or smaller than 20 mm, preferably equal to or smaller than 10 mm, more preferably equal to or smaller than 5 mm. The smaller pitch P₁ gets the shorter current return path that achieves better signal quality.

According to the embodiment, each of the via holes 201 has a diameter that is greater than or equal to 0.1 mm, for example, 0.25 mm. A via land pad 901 is formed on each of the via hole 201. According to the embodiment, the via land pad 901 has a diameter that is greater than or equal to 0.2 mm, for example, 0.5 mm. The larger diameter of via hole 201 gets the smaller ground impedance.

According to the embodiment, the second signal nets 203 b are electrically connected to an overlying conductive layer 207 through the openings 209. Likewise, these openings 209 are arranged along the lengthwise direction of each of the second signal nets 203 b with a pitch P₂. According to the embodiment, the pitch P₂ may be equal to or smaller than 20 mm, preferably equal to or smaller than 10 mm. According to the embodiment, each of the openings 209 covers an area with a diameter that is greater than or equal to 0.1 mm, or an area about 0.008 mm². For example, each of the openings 209 may covers an area with a square shape when viewed from the above and may have a dimension of 0.1 mm×0.1 mm, or an area of 0.01 mm². The smaller pitch P₂ gets the shorter current return path that achieves better signal quality. The larger opening area 209 connected with the conductive layer 207 also gets the smaller ground impedance. In order to shorten the current return path further, the via hole 201 may be exposed in the opening 209 and connected with the conductive layer 207 directly.

The outermost insulating layer 204 is also shown in FIG. 9. The outermost insulating layer 204 covers the high-speed interface 30 including the first signal nets 203 a and the second signal nets 203 b. According to the embodiment, the outermost insulating layer 204 does not cover the first chip 10 and the second chip 20. The conductive layer 207 is disposed on the outermost insulating layer 204. The conductive layer 207 may comprise solid metal sheet, metal foil, metal tape, conductive epoxy, conductive epoxy adhesive, conductive silver, copper, aluminum epoxy, polymer conductive composite, conductive polymer, conductive composite polymer, or carbon (graphite) printing, but is not limited thereto.

As can be seen in FIG. 9, the conductive layer 207 only covers the region W. Therefore, the area outside the region W is not covered by the conductive layer 207. The conductive layer 207 does not cover the first chip 10 and the second chip 20. The conductive layer 207, together with the underlying ground plane and the densely arranged via holes 201 and openings 209, may shorten the current return path of the first signal nets 203 a along their lengthwise direction in the PCB so as to effectively avoid EMI interference or coupling between the high-frequency signal traces on the PCB.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A printed circuit board, comprising: a substrate having a top surface and a bottom surface; a first signal net and a second signal net being in close proximity to each other and disposed within a specific region on the top surface of the substrate; an outermost insulating layer disposed on the top surface of the substrate to cover the substrate, the first signal net and the second signal net, wherein the outmost insulating layer comprises a plurality of openings which expose at least a top portion of the second signal net from the outmost insulating layer, the openings arranged along a lengthwise direction of the second signal net with a first pitch, wherein the first pitch is equal to or smaller than 20 mm; a reference plane disposed underneath the bottom surface of the substrate and underlying the first signal net and the second signal net, wherein the reference plane is electrically connected to the second signal net through a plurality of via holes formed underneath a bottom portion of the second signal net and disposed in the substrate, the via holes arranged along the lengthwise direction of the second signal net with a second pitch, wherein the second pitch is equal to or smaller than 20 mm; and a conductive layer disposed in the openings to electrically connect to the second signal net, and the conductive layer further disposed on the outermost insulating layer corresponding to the specific region in which the first signal net is arranged such that the conductive layer overlaps with the first signal net, wherein positions of the via holes underneath the second signal net interleave with positions of the openings on top of the second signal net along the lengthwise direction of the second signal net.
 2. The printed circuit board according to claim 1, wherein each of the plurality of via holes is a completely-filled via hole.
 3. The printed circuit board according to claim 1, wherein each of the via holes has a diameter that is greater than or equal to 0.1 mm.
 4. The printed circuit board according to claim 3, wherein a via land pad is formed on each of the via hole, and wherein the via land pad has a diameter that is greater than or equal to 0.2 mm.
 5. The printed circuit board according to claim 1, wherein each of the openings covers an area that is greater than or equal to 0.008 mm2. 